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Mr Netanyahu will address the UN General Assembly on Tuesday

Israeli PM Benjamin Netanyahu is heading to the US for talks with President Barack Obama which will focus on Iran's diplomatic charm offensive.

Before leaving the US on Friday, Iran's new President, Hassan Rouhani, shared a 15-minute phone call with Mr Obama.

Mr Netanyahu, who will meet Mr Obama on Monday then address the UN on Tuesday, said he would "tell the truth in the face of the sweet talk".

Israel and the West suspect Iran is trying to build a nuclear bomb.

Tehran denies the claim. Mr Rouhani, who spoke to President Obama late on Friday, has said he wants to seek a deal with world powers on Iran's nuclear programme within months.

The phone conversation was the first top-level conversation between the two countries for more than 30 years. Mr Rouhani, a moderate, was elected in June, replacing hardline President Mahmoud Ahmadinejad.

On his return to Tehran on Saturday, Mr Rouhani was welcomed by hundreds of supporters hailing his trip, as well as a smaller number of opponents chanting "Death to America" and "Death to Israel".

The Israeli government has not officially reacted to the Iranian president's remarks and his historic phone conversation with Mr Obama. Israeli media reported that ministers had been instructed not to speak publicly about the issue, to avoid gaffes before Monday's White House meeting.

But writing on his Facebook page, former Foreign Minister Avigdor Lieberman warned that the Iranian president was engaging in a "deceitful exercise".

Hours after Mr Netanyahu's plane took off for New York, Israel's Shin Bet intelligence service disclosed that a Belgian man of Iranian origin had been arrested on 11 September on suspicion of spying.

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A handout from Israel's Shin Bet agency of a man named as Ali Mansouri

Ali Mansouri, 50, entered Israel under the name Alex Mans and had been sent by Iran's Revolutionary Guard elite Quds force, a Shin Bet statement said.

He was detained at Ben Gurion airport at the end of a five-day trip, during which he had photographed the roof of the US embassy in Tel Aviv, it said.

Tehran insists its nuclear activities are for peaceful purposes but last month the UN's nuclear agency said Iran had further boosted its capacity for uranium enrichment, installing more than 1,000 advanced centrifuges at the Natanz enrichment plant.

Iran is due to take part in substantive negotiations on its nuclear programme in Geneva on 15 October with a group of nations known as the P5+1, which include the US, Russia, China, the UK, France and Germany.

Mr Rouhani has said Iran will bring a plan to that meeting, but has not provided details.

The P5+1 have called on Tehran to halt production and stockpiling of uranium enriched to 20%. Uranium enriched to 90% is required for a nuclear weapon.